1. Field of the Invention
Aspects of the present invention relate to a thin film transistor and a method of fabricating the same. More particularly, aspects of the present invention relate to a thin film transistor including a semiconductor layer with crystal grains of constant directivity to minimize the characteristic dispersion depending on the crystal grain state, and a method of fabricating the same.
2. Description of the Related Art
Cathode-ray tubes (CRT) have been superseded by flat panel display devices (FPD) that can be fabricated to be lightweight and thin. Typical examples of FPDs are a liquid crystal display device (LCD) and an organic light emitting diode (OLED) display device. The OLED display device has a higher luminance and a wider viewing angle than the LCD and may be made ultra-thin, since an OLED display device does not need a backlight.
In an OLED display device, electrons and holes are injected through a cathode and an anode, respectively, and recombine in an organic thin layer to form excitons. The OLED emits light in a specific wavelength range due to energy created by the excitons.
OLED display devices may be classified into a passive matrix (PM) type and an active matrix (AM) type depending on the driving method. The AM-type OLED display device includes a circuit that uses a thin film transistor (TFT). The PM-type OLED display device, on the other hand, does not include a TFT driving circuit and may be easily fabricated since the display region comprises a simple matrix-type arrangement with anodes and cathodes. However, the application range of PM-type OLED display devices is restricted to low-resolution small-sized display devices owing to problems of resolution, a high driving voltage, and a shortened life span of materials. In AM-type OLED display devices, each pixel of the display device includes a TFT, which means that a constant amount of current can be supplied to each pixel to obtain stable luminance. Also, since AM-type OLED display devices consume low power, AM-type OLED display devices can be high-resolution large-sized display devices.
A TFT generally includes a semiconductor layer having a source region, a drain region and a channel region, a gate electrode, a gate insulating layer, a source electrode, and a drain electrode. The semiconductor layer may be formed of polycrystalline silicon (poly-Si) or amorphous silicon (a-Si). However, electron mobility of polycrystalline silicon is higher than that of amorphous silicon, so polycrystalline silicon is usually employed.
Forming a semiconductor layer made of polycrystalline silicon is generally accomplished by forming amorphous silicon on a substrate and then crystallizing the same. Crystallization methods that can be used include solid phase crystallization (SPC), rapid thermal annealing (RTA), metal induced crystallization (MIC), metal induced lateral crystallization (MILC), a crystallization method using a laser, and similar methods.
In a laser crystallization method, a laser beam is turned on for 30 ns to 200 ns to instantaneously melt the amorphous silicon, and then the melted silicon is cooled and crystallized. This method has the advantage that the thermal effect on the substrate can be minimized and a semiconductor layer having good crystallinity can be formed. Two types of laser crystallization methods are excimer laser annealing (ELA) and sequential lateral solidification (SLS), in which a laser beam is typically irradiated on the amorphous silicon at least two times in order to have crystal grains laterally grown and crystallized.
However, the laser crystallization method does not allow for the crystal grains of the polycrystalline silicon to have constant directivity, but rather, crystal grain boundaries between the crystal grains are non-uniformly distributed. Since the crystal grain boundaries affect characteristics such as electron mobility of TFTs using polycrystalline silicon, the characteristic dispersion of the TFT disadvantageously occurs due to the distribution of the non-uniform crystal grain boundaries when the polycrystalline silicon is formed by the laser crystallization method.